This invention relates to improvements in casting rods used in the manufacture of contact lenses and intraocular implants. More particularly, this invention relates to casting polymerization mixtures in Teflon tubes or Teflon-coated tubes.
U.S. Pat. No. 3,978,164, issued to LeBoeuf et al., describes casting contact lens material in a Teflon tube lined with a Mylar or a similar film. Certain materials cannot be thus cast and this invention is directed to an improvement in the casting process providing uniform polymerized rods which cannot be cast using the process of the aforementioned patent. In attempting to overcome some of the difficulties encountered in trying to adopt the method of the aforementioned patent to other materials, we tried glass tubes. The tubes were sealed from the atmosphere by appropriate plugs and nonoxidizing atmospheres. In one instance, we used a nitrogen filled balloon about the open end of the tube in which the monomer was cast. While the rods recovered from such a glass casting system were more uniform in composition, they were not more uniform in geometry, quite surprisingly. The rods recovered were noncircular in that they had flat areas along the length of the rod. Also, the glass tubes seemed to become more fragile with repeated use. While not able to fully understand the phenomenon by which this increase in fragility occurs, we postulate a fatigue was induced in the glass. This may have been due to leaching of minor constituents from the glass, or an accumulation of scratches during handling and cleaning.
In any event, we next tried metal tubes of stainless steel and aluminum. We selected these metal tubes because of the desirability of controlling the exothermic reaction which occurs when the mixture is polymerizing. These metal tubes were also not entirely satisfactory. Among other things, non-circular flat areas formed on the cast rods and there was much sticking. Tubes of polyethylene and other polyolefins known to be "slippery" were tried. Polymerization in the polyolefin tubes did not proceed uniformly. Apparently, inhibitors required for polyolefins adversely affect the polymerization of NVP containing monomers. Also, we tried tubes of Teflon. Teflon is the trademark of the DuPont Company for a waxy opaque material called polytetrafluoroethylene. This material has been used, for example, on cooking utensils and in many industrial applications to prevent sticking. The combination of teflon tubes within a copper or aluminum stiffener proved successful in accomplishing the objects of the invention; that is, the reproducible manufacture of a rod used in hard contact lens and intraocular lens precursor manuacture characterized by good machinability, color uniformity and optical quality. Certain Teflon coatings on metal tubing have also accomplished this same end.